Fishscopes known today are designed to generate an oscilloscopic trace on a screen or TV monitor where the trace is derived from and represents a fish or other target object at a certain depth. Earlier fishscopes utilized electrostatically deflected oscilloscope tubes. But in recent years, fishscopes have moved to using magnetically deflected tubes of the type commonly found in small TV receivers.
Generally then, fishscopes of the prior art employ standard oscilloscope technology inasmuch as the CRT electron beam is slowly deflected along one cartesien axis in order to display depth and is rapidly deflected along the orthogonal axis to display the echo signature trace. It will be appreciated that the echo signature trace can appear as a bilateral wave envelope representing the peak to peak high frequency excursion of the echo signal, or can be rectified and filtered to produce a unilateral trace of the echo profile.
More specifically, in fishscopes of the prior art, one can expect to find the same designed such that the cathode ray tube is magnetically deflected by conventional deflection yokes. To provide horizontal deflection of the electronic beam rapidly enough to present a clean echo image requires that the yoke be fed approximately two amps at 60 volts peak to peak at 15 to 30 kHz. To perform this function, the signal produced by a receiver amplifier at some frequency (X) must be summed in a tuned mixer amplifier with a local oscillator (Y) to produce a difference frequency of say, for example, 20 kHz (Z). This signal must be processed through an additional tuned amplifier to reduce the X and Y frequencies to a point where they have no effect on the output drive amplifier. The output drive amplifier instead of operating between plus and minus 30 volts which would be wasteful of power, normally operates at 12 to 24 volts and is normally transformer or push-pull transistor coupled to the yoke. To further simplify the system, the yoke is series resonated to 20 kHz with a series capacitor to drop the driving voltage to normal buss voltage.
To operate a fishscope as described above requires a substantial amount of expensive equipment such as a separate high voltage power supply, an amplifier to produce proper yoke frequencies, a second filter amplifier, and a power amplifier capable of generating two amps of signal current. In the end, these requirements necessitate a complex and expensive amplifier, an expensive high voltage power supply and a high current power supply to supply the yoke amplifier.
Beyond the above, conventional fishscopes of the prior art have other disadvantages. Among these disadvantages are that under normal circumstances only one echoing system and its attendant trace can be presented at one time. Further, the electronic beam is not normally blanked except during retrace causing a continuous line to appear on the scope face. In addition, as a general rule, no information other than the oscilloscopic trace can be displayed on the CRT tube without extensively modifying the tube construction. It is also known that electrostatically deflected CRT'S are normally dim and require high voltage deflecting potentials. Likewise, magnetically deflected CRT'S requires high yoke current to generate high frequency beam deflection.